Maintaining the benefit of parallel splitting of ops between primary and secondary storage clusters in synchronous replication while adding support for op logging and early engagement of op logging

ABSTRACT

Systems and methods are described for performing persistent inflight tracking of operations (Ops) within a cross-site storage solution. According to one embodiment, a method comprises maintaining state information regarding a data synchronous replication status for a first storage object of a primary storage cluster and a second storage object of a secondary storage cluster. The method includes performing persistent inflight tracking of I/O operations with a first Op log of the primary storage cluster and a second Op log of the secondary storage cluster, establishing and comparing Op ranges for the first and second Op logs, and determining a relation between the Op range of the first Op log and the Op range of the second Op log to prevent divergence of Ops in the first and second Op logs and to support parallel split of the Ops.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No. 17/510,795, filed Oct. 26, 2021, which claims the benefit of Indian Provisional Application No. 202141020578, filed on May 5, 2021, and Indian Provisional Application No. 202141020579, filed on May 5, 2021, which are hereby incorporated by reference in its entirety for all purposes.

COPYRIGHT NOTICE

Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever. Copyright© 2021, NetApp, Inc.

FIELD

Various embodiments of the present disclosure generally relate to multi-site distributed data storage systems. In particular, some embodiments relate to maintaining the benefit of parallel splitting of operations (ops) between primary and secondary storage clusters in synchronous replication while adding support for op logging and early engagement of op logging.

BACKGROUND

Multiple storage nodes organized as a cluster may provide a distributed storage architecture configured to service storage requests issued by one or more clients of the cluster. The storage requests are directed to data stored on storage devices coupled to one or more of the storage nodes of the cluster. The data served by the storage nodes may be distributed across multiple storage units embodied as persistent storage devices, such as hard disk drives (HDDs), solid state drives (SSDs), flash memory systems, or other storage devices. The storage nodes may logically organize the data stored on the devices as volumes accessible as logical units. Each volume may be implemented as a set of data structures, such as data blocks that store data for the volume and metadata blocks that describe the data of the volume.

Business enterprises rely on multiple clusters for storing and retrieving data. Each cluster may be a separate data center with the clusters able to communicate over an unreliable network. The network can be prone to failures leading to connectivity issues such as transient or persistent connectivity issues that disrupt operations of a business enterprise.

SUMMARY

Systems and methods are described for performing persistent inflight tracking of operations (Ops) and resynchronization between storage objects within a cross-site storage solution. According to one embodiment, a method performed by one or more processing resources of a distributed storage system comprises maintaining state information regarding a data synchronous replication status for a first storage object of a primary storage cluster with the storage object being replicated to a second storage object of a secondary storage cluster. The state information facilitates automatic triggering of resynchronization for data replication between the first storage object and the second storage object. The method includes performing persistent inflight tracking of I/O operations with a first Op log of the primary storage cluster and a second Op log of the secondary storage cluster, establishing Op ranges for the first and second Op logs, comparing the Op range of the first Op log of the primary storage cluster and the Op range of the second Op log of the secondary storage cluster, and determining a relation between the Op range of the first Op log and the Op range of the second Op log to prevent divergence of Ops in the first and second Op logs and to support parallel split of the Ops.

Other features of embodiments of the present disclosure will be apparent from accompanying drawings and detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

FIG. 1 is a block diagram illustrating an environment in which various embodiments may be implemented.

FIG. 2 is a block diagram illustrating an environment having potential failures within a multi-site distributed storage system in which various embodiments may be implemented.

FIG. 3 is a block diagram of a multi-site distributed storage system according to various embodiments of the present disclosure.

FIG. 4 is a block diagram illustrating a storage node in accordance with an embodiment of the present disclosure.

FIG. 5 is a block diagram illustrating the concept of a consistency group (CG) in accordance with an embodiment of the present disclosure.

FIG. 6A is a CG state diagram in accordance with an embodiment of the present disclosure.

FIG. 6B is a volume state diagram in accordance with an embodiment of the present disclosure.

FIG. 7 is a block diagram illustrating a persistent inflight tracker that is implemented with operation (Op) logs in a distributed storage system in accordance with an embodiment of the present disclosure.

FIG. 8 is a flow diagram illustrating a set of operations for performing a persistent inflight tracking replay in accordance with an embodiment of the present disclosure.

FIG. 9 is a flow diagram illustrating a set of operations for performing a persistent inflight tracking replay and reconciliation in accordance with an embodiment of the present disclosure.

FIG. 10 is a flow diagram illustrating a set of operations for comparing ranges of Op logs for primary and secondary storage clusters while performing a persistent inflight tracking replay in accordance with an embodiment of the present disclosure.

FIG. 11 is a flow diagram illustrating a set of operations for early engagement of Op logs for primary and secondary storage clusters while marking the Op log file usable later to reduce user I/O latency in accordance with an embodiment of the present disclosure.

FIG. 12 is a flow diagram illustrating a set of operations for early engagement of Op logs for primary and secondary storage clusters while marking the Op log file usable later to reduce user I/O latency in accordance with an embodiment of the present disclosure.

FIG. 13 illustrates a synchronous replication (SR) system with tracker updates for a transition or resynchronization process in accordance with one embodiment.

FIG. 14 illustrates parallel splitting of data Ops and sequentially splitting of modifying metadata Ops for a multi-site distributed storage system having synchronous replication (SR) between primary and secondary sites in accordance with one embodiment.

FIG. 15 illustrates an example computer system in which or with which embodiments of the present disclosure may be utilized.

DETAILED DESCRIPTION

Systems and methods are described for maintaining the benefit of parallel splitting of operations (ops) between primary and secondary storage clusters in synchronous replication while adding support for op logging and early engagement of op logging.

In the context of cross-site distributed storage system (including cross-site HA storage solutions that perform synchronous data replication to support zero RPO protection), a certain degree of consistency over time is maintained between a mirror copy and a primary dataset depending upon the particular implementation. Certain operations on a storage object (e.g., data container/volume) of a consistency group (CG) having numerous data containers/volumes hosting the data at issue should be managed independently for an out of sync storage object in order to avoid transitioning all storage objects of a CG out of sync and avoid delays needed for transitioning all storage objects with resynchronization from out of sync back in sync.

In one example, a primary and a secondary storage cluster are diverged due to inflight I/O operations (ops) that are not yet acknowledged to a client device. An inflight op is an op that is in progress on either primary or secondary storage cluster and its response is held by a synchronous replication circuitry (SR circuitry), which includes a splitter component and a file system. An inflight Op can be a data Op (e.g., write, punch hole, etc.) or a metadata op (e.g., create, unlink, set attribute, etc.). An inflight Op can have the following states:

-   -   request received: Op is received by splitter component     -   primary sent: Op is in progress with file system of SR circuitry         of primary storage cluster     -   primary done: Op completed by file system of primary storage         cluster and splitter component callback has been called.     -   secondary sent: Op is dispatched to secondary storage cluster         and is either in transit (network) or in progress by file system         of SR circuitry of secondary storage cluster.     -   secondary done: Op completed by file system of secondary storage         cluster and splitter component has received a completion         callback.     -   response sent: Op completed on both of primary and secondary         storage cluster and a response is released by splitter component         of SR circuitry.

A splitter component can include a queue to store incoming operations and a splitter object that is configured to split (replicate) operations targeting a storage object. The splitter object replicates the operations to a replicated storage object of the second storage cluster. Operations that been acknowledged to the client device have been executed by a storage cluster and hence committed on both primary and secondary endpoints for the primary and secondary storage clusters. However, at a given instance of time, one or more Ops could be in flight i.e., executed on neither of endpoints (e.g., first storage object hosted by primary storage cluster, replicated second storage object hosted by secondary storage cluster), both of the endpoints, or executed on one of the endpoints. As a consequence, the primary and second storage clusters at a given point in time could be divergent with respect to inflight Ops. A common snapshot may be performed periodically to serve as resynchronization points.

Data operations are designed with an idempotent property while metadata operations are designed with a non-idempotent property. To address this divergence, the present design when in a state of synchronous replication (in sync state) will persistently track inflight operations. Also, before opening up a storage object for I/O operations of a client device, the present design will replace the inflight operations prior to resuming synchronous replication.

In another example, the present design in order to maintain a secondary storage cluster as being failover capable will cause an out of sync (OOS) event for a data container/volume to be an internal event and will also avoid write order inconsistency. The present design disallows I/O operations on the OOS data container/volume until resumption of synchronous replication and this avoids dependent write order inconsistencies. The present design performs a fast establishment of a transfer engine for resynchronization and keeps the CG in sync while one or more data containers/volume in the CG having an internal event to indicate OOS.

Embodiments described herein seek to improve various technological processes associated with cross-site storage solutions and ensure the process of quickly establishing resynchronization between a storage object (e.g., first storage object) of a primary storage cluster and a replicated storage object (e.g., second storage object) of a secondary storage cluster. Various embodiments of the present technology provide for a wide range of technical effects, advantages, and/or improvements to stretched storage systems and participating distributed storage systems. For example, various embodiments may include one or more of the following technical effects, advantages, and/or improvements: (i) provide an Op log on a primary storage cluster and another Op log on the secondary copy. Both Op logs will specify which operations are committed on each of the storage clusters. These two Op logs can be used to find how the two filesystems are differing and carry out resynchronization; (ii) maintain the benefit of parallel splitting of Ops to primary and secondary storage clusters in synchronous replication while adding support for Op logging on both primary and secondary storage clusters; and (iii) start Op logging early and marking the Op log usable once the synchronous replication relationship between a first storage object and a replicated second storage object enters In Sync state. Early engagement of Op logging avoids I/O latency for a client device. One or more of which may include various additional optimizations described further below.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent, however, to one skilled in the art that embodiments of the present disclosure may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form.

Terminology

Brief definitions of terms used throughout this application are given below.

A “computer” or “computer system” may be one or more physical computers, virtual computers, or computing devices. As an example, a computer may be one or more server computers, cloud-based computers, cloud-based cluster of computers, virtual machine instances or virtual machine computing elements such as virtual processors, storage and memory, data centers, storage devices, desktop computers, laptop computers, mobile devices, or any other special-purpose computing devices. Any reference to “a computer” or “a computer system” herein may mean one or more computers, unless expressly stated otherwise.

The terms “connected” or “coupled” and related terms are used in an operational sense and are not necessarily limited to a direct connection or coupling. Thus, for example, two devices may be coupled directly, or via one or more intermediary media or devices. As another example, devices may be coupled in such a way that information can be passed there between, while not sharing any physical connection with one another. Based on the disclosure provided herein, one of ordinary skill in the art will appreciate a variety of ways in which connection or coupling exists in accordance with the aforementioned definition.

If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.

As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

The phrases “in an embodiment,” “according to one embodiment,” and the like generally mean the particular feature, structure, or characteristic following the phrase is included in at least one embodiment of the present disclosure, and may be included in more than one embodiment of the present disclosure. Importantly, such phrases do not necessarily refer to the same embodiment.

Example Operating Environment

FIG. 1 is a block diagram illustrating an environment 100 in which various embodiments may be implemented. In various examples described herein, an administrator (e.g., user 112) of a multi-site distributed storage system 102 having clusters 135 and cluster 145 or a managed service provider responsible for multiple distributed storage systems of the same or multiple customers may monitor various operations and network conditions of the distributed storage system or multiple distributed storage systems via a browser-based interface presented on computer system 110.

In the context of the present example, the multi-site distributed storage system 102 includes a data center 130, a data center 140, and optionally a mediator 120. The data centers 130 and 140, the mediator 120, and the computer system 110 are coupled in communication via a network 105, which, depending upon the particular implementation, may be a Local Area Network (LAN), a Wide Area Network (WAN), or the Internet.

The data centers 130 and 140 may represent an enterprise data center (e.g., an on-premises customer data center) that is owned and operated by a company or the data center 130 may be managed by a third party (or a managed service provider) on behalf of the company, which may lease the equipment and infrastructure. Alternatively, the data centers 130 and 140 may represent a colocation data center in which a company rents space of a facility owned by others and located off the company premises. The data centers are shown with a cluster (e.g., cluster 135, cluster 145). Those of ordinary skill in the art will appreciate additional IT infrastructure may be included within the data centers 130 and 140. In one example, the data center 140 is a mirrored copy of the data center 130 to provide non-disruptive operations at all times even in the presence of failures including, but not limited to, network disconnection between the data centers 130 and 140 and the mediator 120, which can also be located at a data center.

Turning now to the cluster 135, it includes multiple storage nodes 136 a-n and an Application Programming Interface (API) 137. In the context of the present example, the multiple storage nodes 136 a-n are organized as a cluster and provide a distributed storage architecture to service storage requests issued by one or more clients (not shown) of the cluster. The data served by the storage nodes 136 a-n may be distributed across multiple storage units embodied as persistent storage devices, including but not limited to HDDs, SSDs, flash memory systems, or other storage devices. In a similar manner, cluster 145 includes multiple storage nodes 146 a-n and an Application Programming Interface (API) 147. In the context of the present example, the multiple storage nodes 146 a-n are organized as a cluster and provide a distributed storage architecture to service storage requests issued by one or more clients of the cluster.

The API 137 may provide an interface through which the cluster 135 is configured and/or queried by external actors (e.g., the computer system 110, data center 140, the mediator 120, clients). Depending upon the particular implementation, the API 137 may represent a Representational State Transfer (REST)ful API that uses Hypertext Transfer Protocol (HTTP) methods (e.g., GET, POST, PATCH, DELETE, and OPTIONS) to indicate its actions. Depending upon the particular embodiment, the API 137 may provide access to various telemetry data (e.g., performance, configuration, storage efficiency metrics, and other system data) relating to the cluster 135 or components thereof. As those skilled in the art will appreciate various other types of telemetry data may be made available via the API 137, including, but not limited to measures of latency, utilization, and/or performance at various levels (e.g., the cluster level, the storage node level, or the storage node component level).

In the context of the present example, the mediator 120, which may represent a private or public cloud accessible (e.g., via a web portal) to an administrator associated with a managed service provider and/or administrators of one or more customers of the managed service provider, includes a cloud-based, monitoring system.

While for sake of brevity, only two data centers are shown in the context of the present example, it is to be appreciated that additional clusters owned by or leased by the same or different companies (data storage subscribers/customers) may be monitored and one or more metrics may be estimated based on data stored within a given level of a data store in accordance with the methodologies described herein and such clusters may reside in multiple data centers of different types (e.g., enterprise data centers, managed services data centers, or colocation data centers).

FIG. 2 is a block diagram illustrating an environment 200 having potential failures within a multi-site distributed storage system 202 in which various embodiments may be implemented. In various examples described herein, an administrator (e.g., user 212) of a multi-site distributed storage system 202 having clusters 235 and cluster 245 or a managed service provider responsible for multiple distributed storage systems of the same or multiple customers may monitor various operations and network conditions of the distributed storage system or multiple distributed storage systems via a browser-based interface presented on computer system 210.

In the context of the present example, the system 202 includes data center 230, data center 240, and optionally a mediator 220. The data centers 230 and 240, the mediator 220, and the computer system 210 are coupled in communication via a network 205, which, depending upon the particular implementation, may be a Local Area Network (LAN), a Wide Area Network (WAN), or the Internet.

The data centers 230 and 240 may represent an enterprise data center (e.g., an on-premises customer data center) that is owned and operated by a company or the data center 230 may be managed by a third party (or a managed service provider) on behalf of the company, which may lease the equipment and infrastructure. Alternatively, the data centers 230 and 240 may represent a colocation data center in which a company rents space of a facility owned by others and located off the company premises. The data centers are shown with a cluster (e.g., cluster 235, cluster 245). Those of ordinary skill in the art will appreciate additional IT infrastructure may be included within the data centers 230 and 240. In one example, the data center 240 is a mirrored copy of the data center 230 to provide non-disruptive operations at all times even in the presence of failures including, but not limited to, network disconnection between the data centers 230 and 240 and the mediator 220, which can also be a data center.

The system 202 can utilize communications 290 and 291 to synchronize a mirrored copy of data of the data center 240 with a primary copy of the data of the data center 230. Either of the communications 290 and 291 between the data centers 230 and 240 may have a failure 295. In a similar manner, a communication 292 between data center 230 and mediator 220 may have a failure 296 while a communication 293 between the data center 240 and the mediator 220 may have a failure 297. If not responded to appropriately, these failures whether transient or permanent have the potential to disrupt operations for users of the distributed storage system 202. In one example, communications between the data centers 230 and 240 have approximately a 5-20 millisecond round trip time.

Turning now to the cluster 235, it includes at least two storage nodes 236 a-b, optionally includes additional storage nodes (e.g., 236 n) and an Application Programming Interface (API) 237. In the context of the present example, the multiple storage nodes are organized as a cluster and provide a distributed storage architecture to service storage requests issued by one or more clients of the cluster. The data served by the storage nodes may be distributed across multiple storage units embodied as persistent storage devices, including but not limited to HDDs, SSDs, flash memory systems, or other storage devices.

Turning now to the cluster 245, it includes at least two storage nodes 246 a-b, optionally includes additional storage nodes (e.g., 246 n) and includes an Application Programming Interface (API) 247. In the context of the present example, the multiple storage nodes are organized as a cluster and provide a distributed storage architecture to service storage requests issued by one or more clients of the cluster. The data served by the storage nodes may be distributed across multiple storage units embodied as persistent storage devices, including but not limited to HDDs, SSDs, flash memory systems, or other storage devices.

In one example, each cluster can have up to 5 CGs with each CG having up to 12 volumes. The system 202 provides a planned failover feature at a CG granularity. The planned failover feature allows switching storage access from a primary copy of the data center 230 to a mirror copy of the data center 240 or vice versa.

FIG. 3 is a block diagram illustrating a multi-site distributed storage system 300 in which various embodiments may be implemented. In various examples described herein, an administrator (e.g., user 313) of the multi-site distributed storage system 300 or a managed service provider responsible for multiple distributed storage systems of the same or multiple customers may monitor various operations and network conditions of the distributed storage system or multiple distributed storage systems via a browser-based interface presented on computer system 308. In the context of the present example, the distributed storage system 300 includes a data center 302 having a cluster 310, a data center 304 having a cluster 320, and a mediator 360. The clusters 310, 320, and the mediator 360 are coupled in communication (e.g., communications 340-342) via a network, which, depending upon the particular implementation, may be a Local Area Network (LAN), a Wide Area Network (WAN), or the Internet. The communications 340-342 provide redundance communication channels for operations of the distributed storage system 300 (e.g., liveliness operation, consensus operation)

The cluster 310 includes nodes 311 and 312 while the cluster 320 includes nodes 321 and 322. In one example, the cluster 320 has a data copy 330 in node 321 that is a mirrored copy of data copy 330 in node 311. A data copy 331 in node 322 is a mirrored copy of the data copy 331 in node 312 to provide non-disruptive operations at all times even in the presence of failures including, but not limited to, network disconnection between the data centers 302 and 304 and the mediator 360.

The multi-site distributed storage system 300 provides correctness of data, availability, and redundancy of data. In one example, the nodes 311 and 312 are designated as a master and the nodes 321 and 322 are designated as a slave. The master is given preference to serve I/O commands to requesting clients and this allows the master to obtain a consensus in a case of a race between the clusters 310 and 320. The mediator 360 enables an automated unplanned failover (AUFO) in the event of a failure. The data copy 330 (master), data copy 331 (slave), and the mediator 360 form a three way quorum. If two of the three entities reach an agreement for whether the master or slave should serve I/O commands to requesting clients, then this forms a strong consensus.

In one embodiment, node 311 has a failure and the data copy 331 for a storage object of node 312 remains in sync. The node 312 handles a takeover operation for data copy 330 (master). Upon a volume mount time, the node 311 temporarily disallows input/output operations (e.g., both read and write) with a retriable error. The I/O operations from a computer system 310 are not allowed at node 311 until resynchronization occurs or a timeout occurs.

Next, the cluster 320 performs an automatic Fast Resynchronization (Fast Resync) to maintain zero recovery point objective (RPO) protection. The Fast Resync is based on reestablishing a Sync Data Path between data copy 330 (master) of node 311 and data copy 330 (slave) of mirrored node 321, and reconciling inflight regions based on persistent inflight tracking of I/O operations (IFT-P). The secondary storage cluster 320 can be provided with necessary information about a high availability partner to avoid cross-cluster calls between the primary and secondary storage cluster. Note, no asynchronous transfers and transition are allowed during the Fast Resync, which will establish a transfer engine session and start persistent inflight op tracking replay. A Fast Resync can be triggered as soon a storage object on the secondary storage cluster is mounted.

Subsequently, node 311 waits for resumption of synchronous replication and allows I/O upon completion of the synchronous replication.

If Fast Resync experiences an error or failure resulting in the Fast Resync not being possible within a certain time period (e.g., 30-90 seconds, 60 seconds), then the following phases occur:

Phase 1: After expiration of the certain time period, node 311 will take a CG for node 311 out of sync (OOS). The state diagrams for the CG and a storage object (e.g., data container/volume) are illustrated in FIGS. 6A and 6B when Fast Resync has an error or failure.

Phase 2: add a strict sync policy to database software management that will disallow I/O for an extended time period or indefinite time period. Phase 1 behavior will be the default mode of operations if fast resync is not successfully performed within the certain time period.

The master and slave roles for the clusters 310 and 320 help to avoid a split-brain situation with both of the clusters simultaneously attempting to serve I/O commands. There are scenarios where both master and slave copies can claim to be a master copy. For example, a recovery post failover or failure during planned failover workflow can results in both clusters 310 and 320 attempting to serve I/O commands. In one example, a slave cannot serve I/O until an AUFO happens. A master doesn't serve I/O commands until the master obtains a consensus.

The multi-site distributed storage system 300 presents a single virtual logical unit number (LUN) to a host computer or client using a synchronized-replicated distributed copies of a LUN. A LUN is a unique identifier for designating an individual or collection of physical or virtual storage devices that execute input/output (I/O) commands with a host computer, as defined by the Small System Computer Interface (SCSI) standard. In one example, active or passive access to this virtual LUN causes read and write commands to be serviced only by node 311 (master) while operations received by the node 321 (slave) are proxied to node 311.

Example Storage Node

FIG. 4 is a block diagram illustrating a storage node 400 in accordance with an embodiment of the present disclosure. Storage node 400 represents a non-limiting example of storage nodes (e.g., 136 a-n, 146 a-n, 236 a-n, 246 a-n, 311, 312, 331, 322, 712, 715, 752, 754) described herein. In the context of the present example, storage node 400 includes a storage operating system 410, one or more slice services 420 a-n, and one or more block services 415 a-q. The storage operating system (OS) 410 may provide access to data stored by the storage node 400 via various protocols (e.g., small computer system interface (SCSI), Internet small computer system interface (ISCSI), fibre channel (FC), common Internet file system (CIFS), network file system (NFS), hypertext transfer protocol (HTTP), web-based distributed authoring and versioning (WebDAV), or a custom protocol. A non-limiting example of the storage OS 410 is NetApp Element Software (e.g., the SolidFire Element OS) based on Linux and designed for SSDs and scale-out architecture with the ability to expand up to 100 storage nodes.

Each slice service 420 may include one or more volumes (e.g., volumes 421 a-x, volumes 421 c-y, and volumes 421 e-z). Client systems (not shown) associated with an enterprise may store data to one or more volumes, retrieve data from one or more volumes, and/or modify data stored on one or more volumes.

The slice services 420 a-n and/or the client system may break data into data blocks. Block services 415 a-q and slice services 420 a-n may maintain mappings between an address of the client system and the eventual physical location of the data block in respective storage media of the storage node 400. In one embodiment, volumes 421 include unique and uniformly random identifiers to facilitate even distribution of a volume's data throughout a cluster (e.g., cluster 135). The slice services 420 a-n may store metadata that maps between client systems and block services 415. For example, slice services 420 may map between the client addressing used by the client systems (e.g., file names, object names, block numbers, etc. such as Logical Block Addresses (LBAs)) and block layer addressing (e.g., block IDs) used in block services 415. Further, block services 415 may map between the block layer addressing (e.g., block identifiers) and the physical location of the data block on one or more storage devices. The blocks may be organized within bins maintained by the block services 415 for storage on physical storage devices (e.g., SSDs).

As noted above, a bin may be derived from the block ID for storage of a corresponding data block by extracting a predefined number of bits from the block identifiers. In some embodiments, the bin may be divided into buckets or “sublists” by extending the predefined number of bits extracted from the block identifier. A bin identifier may be used to identify a bin within the system. The bin identifier may also be used to identify a particular block service 415 a-q and associated storage device (e.g., SSD). A sublist identifier may identify a sublist with the bin, which may be used to facilitate network transfer (or syncing) of data among block services in the event of a failure or crash of the storage node 400. Accordingly, a client can access data using a client address, which is eventually translated into the corresponding unique identifiers that reference the client's data at the storage node 400.

For each volume 421 hosted by a slice service 420, a list of block IDs may be stored with one block ID for each logical block on the volume. Each volume may be replicated between one or more slice services 420 and/or storage nodes 400, and the slice services for each volume may be synchronized between each of the slice services hosting that volume. Accordingly, failover protection may be provided in case a slice service 420 fails, such that access to each volume may continue during the failure condition.

Consistency Groups

FIG. 5 is a block diagram illustrating the concept of a consistency group (CG) in accordance with an embodiment of the present disclosure. In the context of the present example, a stretch cluster including two clusters (e.g., cluster 110 a and 110 b) is shown. The clusters may be part of a cross-site high-availability (HA) solution that supports zero recovery point objective (RPO) and zero recovery time objective (RTO) protections by, among other things, providing a mirror copy of a dataset at a remote location, which is typically in a different fault domain than the location at which the dataset is hosted. For example, cluster 110 a may be operable within a first site (e.g., a local data center) and cluster 110 b may be operable within a second site (e.g., a remote data center) so as to provide non-disruptive operations even if, for example, an entire data center becomes non-functional, by seamlessly failing over the storage access to the mirror copy hosted in the other data center.

According to some embodiments, various operations (e.g., data replication, data migration, data protection, failover, and the like) may be performed at the level of granularity of a CG (e.g., CG 115 a or CG 115 b). A CG is a collection of storage objects or data containers (e.g., volumes) within a cluster that are managed by a Storage Virtual Machine (e.g., SVM 111 a or SVM 111 b) as a single unit. In various embodiments, the use of a CG as a unit of data replication guarantees a dependent write-order consistent view of the dataset and the mirror copy to support zero RPO and zero RTO. CGs may also be configured for use in connection with taking simultaneous snapshot images of multiple volumes, for example, to provide crash-consistent copies of a dataset associated with the volumes at a particular point in time. The level of granularity of operations supported by a CG is useful for various types of applications. As a non-limiting example, consider an application, such as a database application, that makes use of multiple volumes, including maintaining logs on one volume and the database on another volume.

The volumes of a CG may span multiple disks (e.g., electromechanical disks and/or SSDs) of one or more storage nodes of the cluster. A CG may include a subset or all volumes of one or more storage nodes. In one example, a CG includes a subset of volumes of a first storage node and a subset of volumes of a second storage node. In another example, a CG includes a subset of volumes of a first storage node, a subset of volumes of a second storage node, and a subset of volumes of a third storage node. A CG may be referred to as a local CG or a remote CG depending upon the perspective of a particular cluster. For example, CG 115 a may be referred to as a local CG from the perspective of cluster 110 a and as a remote CG from the perspective of cluster 110 b. Similarly, CG 115 a may be referred to as a remote CG from the perspective of cluster 110 b and as a local CG from the perspective of cluster 110 b. At times, the volumes of a CG may be collectively referred to herein as members of the CG and may be individually referred to as a member of the CG. In one embodiment, members may be added or removed from a CG after it has been created.

A cluster may include one or more SVMs, each of which may contain data volumes and one or more logical interfaces (LIFs) (not shown) through which they serve data to clients. SVMs may be used to securely isolate the shared virtualized data storage of the storage nodes in the cluster, for example, to create isolated partitions within the cluster. In one embodiment, an LIF includes an Internet Protocol (IP) address and its associated characteristics. Each SVM may have a separate administrator authentication domain and can be managed independently via a management LIF to allow, among other things, definition and configuration of the associated CGs.

In the context of the present example, the SVMs make use of a configuration database (e.g., replicated database (RDB) 112 a and 112 b), which may store configuration information for their respective clusters. A configuration database provides cluster wide storage for storage nodes within a cluster. The configuration information may include relationship information specifying the status, direction of data replication, relationships, and/or roles of individual CGs, a set of CGs, members of the CGs, and/or the mediator. A pair of CGs may be said to be “peered” when one is protecting the other. For example, a CG (e.g., CG 115 b) to which data is configured to be synchronously replicated may be referred to as being in the role of a destination CG, whereas the CG (e.g., CG 115 a) being protected by the destination CG may be referred to as the source CG. Various events (e.g., transient or persistent network connectivity issues, availability/unavailability of the mediator, site failure, and the like) impacting the stretch cluster may result in the relationship information being updated at the cluster and/or the CG level to reflect changed status, relationships, and/or roles.

While in the context of various embodiments described herein, a volume of a CG may be described as performing certain actions (e.g., taking other members of a CG out of synchronization, disallowing/allowing access to the dataset or the mirror copy, issuing consensus protocol requests, etc.), it is to be understood such references are shorthand for an SVM or other controlling entity, managing or containing the volume at issue, performing such actions on behalf of the volume.

While in the context of various examples described herein, data replication may be described as being performed in a synchronous manner between a paired set of CGs associated with different clusters (e.g., from a primary or master cluster to a secondary or slave cluster), data replication may also be performed asynchronously and/or within the same cluster. Similarly, a single remote CG may protect multiple local CGs and/or multiple remote CGs may protect a single local CG. For example, a local CG can be setup for double protection by two remote CGs via fan-out or cascade topologies. In addition, those skilled in the art will appreciate a cross-site high-availability (HA) solution may include more than two clusters, in which a mirrored copy of a dataset of a primary (master) cluster is stored on more than one secondary (slave) cluster.

FIG. 6A is a CG state diagram 600 in accordance with an embodiment of the present disclosure. In the context of the present example, the data replication status of a CG can generally be in either of an InSync state (e.g., InSync 610) or an OOS state (e.g., OOS 620). Within the OOS state, two sub-states are shown, a not ready for resync state 621 and a ready for resync state 623.

While a given CG is in the InSync state, the mirror copy of the primary dataset associated with the member volumes of the given CG may be said to be in-synchronization with the primary dataset and asynchronous data replication or synchronous data replication, as the case may be, are operating as expected. When a given CG is in the OOS state, the mirror copy of the primary dataset associated with the member volumes of the given CG may be said to be out-of-synchronization with the primary dataset and asynchronous data replication or synchronous data replication, as the case may be, are unable to operate as expected. Information regarding the current state of the data replication status of a CG may be maintained in a configuration database (e.g., RDB 512 a or 512 b).

As noted above, in various embodiments described herein, the members (e.g., volumes) of a CG are managed as a single unit. In the context of the present example, the data replication status of a given CG is dependent upon the data replication status of the individual member volumes of the CG. A given CG may transition 611 from the InSync state to the not ready for resync state 621 of the OOS state responsive to any member volume of the CG becoming OOS with respect to a peer volume with which the member volume is peered. A given CG may transition 622 from the not ready for resync state 621 to the ready for resync state 623 responsive to all member volumes being available. In order to support recovery from, among other potential disruptive events, manual planned disruptive events (e.g., balancing of CG members across a cluster) a resynchronization process is provided to promptly bring the CG back into the InSync state from the OOS state. Responsive to a successful CG resync, a given CG may transition 624 from the ready for resync state 623 to the InSync state.

Although outside the scope of the present disclosure, for completeness it is noted that additional state transitions may exist. For example, in some embodiments, a given CG may transition from the ready for resync state 623 to the not ready for resync state 621 responsive to unavailability of a mediator (e.g., mediator 120) configured for the given CG. In such an embodiment, the transition 622 from the not ready for resync state 621 to the ready for resync state 623 should additionally be based on the communication status of the mediator being available.

FIG. 6B is a volume state diagram 650 in accordance with an embodiment of the present disclosure. In the context of the present example, the data replication status of a volume can be in either of an InSync state (e.g., InSync 630) or an OOS state (e.g., OOS 640). While a given volume of a local CG (e.g., CG 515 a) is in the InSync state, the given volume may be said to be in-synchronization with a peer volume of a remote CG (e.g., CG 515 b) and the given volume and the peer volume are able to communicate with each other via the potentially unreliable network (e.g., network 205), for example, through their respective LIFs. When a given volume of the local CG is in the OOS state, the given volume may be said to be out-of-synchronization with the peer volume of the remote CG and the given volume and the peer volume are unable to communicate with each other. According to one embodiment, a periodic health check task may continuously monitor the ability to communicate between a pair of peered volumes. Information regarding the current state of the data replication status of a volume may be maintained in a configuration database (e.g., RDB 512 a or 512 b).

A given volume may transition 631 from the InSync state to the OOS state responsive to a peer volume being unavailable. A given volume may transition 64 from the OOS state to the InSync state responsive to a successful resynchronization with the peer volume. As described below in further detail, in one embodiment, dependent write-order consistency of the mirror copy is preserved by being responsive to any member volume of a given CG detecting it has gone OOS for any reason (e.g., a network failure), driving all member volumes OOS.

Fast Resynchronization

FIG. 7 is a block diagram illustrating a persistent inflight tracker that is implemented with operation (Op) logs in a distributed storage system in accordance with an embodiment of the present disclosure. In one embodiment, the distributed storage system 700 includes a primary storage cluster 710 with a primary copy of data in a consistency group (CG) that may include a subset or all volumes or data containers of a storage node. The consistency group can include volumes of nodes 712 and 715. Initially, this CG can be assigned a master role. The secondary storage cluster 720 has a mirror copy of the data in a consistency group that may include volumes of nodes 752 and 754. This CG can be initially assigned a slave role.

Each storage cluster may include a configuration database (e.g., persistent replicated database (RDB) 717, 719, RDB 512 a, RDB 512 b,), which is available on all storage nodes of a storage cluster. Each storage cluster includes synchronization replication circuitry (SR circuitry) 713 and 714 for synchronous replication between the storage clusters.

The operation logs or journals synchronize across a filesystem from a primary storage cluster 710 having a primary copy of data and secondary storage cluster 720 having a mirror copy of the data. In the event of an Out of Sync state for a volume due to a network glitch or a node crash, etc., a mechanism is designed to protect data for the volume and its mirror copy, avoid a coordinated OOS state for other volumes within the same CG as the OOS volume, and also avoid an OOS notification from nodes of the secondary storage cluster acting as a slave to an external mediator.

Embodiments of the present disclosure provide an Op log 741 on primary copy of node 712 and another Op log 742 on the secondary copy of node 722. Both of the copies will specify which operations are committed on each of the sides of the storage clusters. These two copies can be used to find how the filesystems for each storage cluster are differing and carry out resynchronzation if necessary. Embodiments of the present disclosure eliminate design options that involve the synchronization replication circuitry (SR) components 713 and 714 directly accessing non-volatile memory contents.

In one example, persistent inflight tracking uses only In-Volume metafiles and has a minimal impact on the Op path length. A Write to metafile isn't logged in memory (e.g., non-volatile memory). Instead a non-volatile log replay of the Op regenerates the entry in the metafile.

The SR circuitry 713 includes an active Ops log file 740 and can be implemented with a circular array. For each Op, the SR circuitry 713 specifies its view of an Inflight Op range <Head, Tail> in a message payload. Even though responses come out of order, the SR circuitry 713 waits for a head Op to be completed and frees up all consecutive Ops which are responded to next.

The various nodes (e.g., storage nodes 136 a-n and storage node 400 ) of the distributed storage systems described herein, and the processing described below with reference to the flow diagrams of FIGS. 8-11 may be implemented in the form of executable instructions stored on a machine readable medium and executed by a processing resource (e.g., a microcontroller, a microprocessor, central processing unit core(s), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), and the like) and/or in the form of other types of electronic circuitry. For example, the processing may be performed by one or more virtual or physical computer systems of various forms, such as the computer system described with reference to FIG. 15 below.

FIG. 8 is a flow diagram illustrating a set of operations for performing a persistent inflight tracking replay in accordance with an embodiment of the present disclosure. In the context of the present example, it is assumed a peered pair of CGs, including a local CG (e.g., CG 515 a) of a distributed storage system (e.g., cluster 510 a) and a peered remote CG (e.g., CG 515 b). One of the volumes of the local CG may have a failure and have an OOS state (e.g., OOS 620) for a period of time. Depending upon the nature of the data replication being performed (e.g., asynchronous or synchronous) and the particular implementation, a resynchronization may be performed responsive to any volume having OOS state.

State information regarding a data replication status of a mirror copy of a dataset associated with a local CG may be maintained, for example, to facilitate automatic triggering of resynchronization. For example, the state information may include information relating to the current availability or unavailability of a peer volume of a local CG.

At operation 802, computer implemented method 800 may initiate a resynchronization process due to a failure of a storage object (e.g., a first storage object) of a first node of a primary storage cluster with the storage object becoming out of sync. A second node of the primary cluster can remain in sync state and handle operations for the first node. At operation 804, the computer implemented method establishes or activates a transfer engine session for resynchronization from the storage object to a mirrored storage object (e.g., second storage object) of a secondary storage cluster where a data copy of the storage object with a failure will be moved to a node of this mirrored storage object. The storage object is not allowed to process I/O operations during this failure. A connectivity loss for the storage object of the first node is treated as an internal state while maintaining an external state as in sync in order to provide time for the transfer engine session to be established for reestablishing synchronous replication within duration of an Op timeout.

In one example, the internal state for OOS does not cause the storage object to generate an out of sync (OOS) event for processing of a mediator or external entity. The internal state of OOS is with respect to the first node and the primary storage cluster while the external state is with respect to any external entities outside of the primary storage cluster. During this internal state with OOS, no user I/O operations are allowed on the storage object that is OOS. If the mediator or a controlling external entity views the storage object as being in sync, then the secondary storage cluster is capable of handling I/O operations for an application (e.g., database application) if the primary storage cluster fails during a resynchronization.

In a different solution, for asynchronous replication between the storage object and the mirrored storage object, the storage object will generate an external OOS event that is sent to a mediator or external entity for processing. For the OOS state, an automatic failover is disallowed.

At operation 806, the computer implemented method waits for a filesystem log replay to finish for an endpoint of the resynchronization. The endpoint can be a storage object of the primary storage cluster. At operation 808, the primary storage cluster obtains content from an active Op log of a node of the secondary storage cluster, starts a persistent inflight tracker (IFT-P) replay of Ops, and waits for this replay to issue all Ops needed for resynchronization between the primary and secondary storage clusters. At operation 810, the primary storage cluster waits for completion of the IFT-P replay and allows new Ops onto the storage object of the primary storage cluster.

FIG. 9 is a flow diagram illustrating a set of operations for performing a persistent inflight tracking replay and reconciliation in accordance with an embodiment of the present disclosure. In the context of the present example, it is assumed a peered pair of CGs, including a local CG (e.g., CG 515 a) of a distributed storage system (e.g., cluster 510 a) and a peered remote CG (e.g., CG 515 b). One of the storage objects (e.g., data containers, volumes) of the local CG may have a failure and have an OOS state (e.g., OOS 620) for a period of time. Depending upon the nature of the data replication being performed (e.g., asynchronous or synchronous) and the particular implementation, a resynchronization may be performed responsive to any storage object having OOS state.

At operation 902, a computer implemented method 900 establishes an Op range based on Op log files from the primary and secondary storage clusters. A comparison between Op ranges of both of the Op log files from the primary and secondary storage clusters is performed to establish the Op range as discussed in conjunction with FIG. 10 below.

At operation 904, the computer implemented method obtains a next Op in the Op range to be considered. At operation 906, the primary storage cluster determines whether the Op being considered is present or absent in both of the Op log files from the primary and second storage clusters. If the Op is present or absent in both of the Op log files from the primary and second storage clusters, then this Op is skipped at operation 908 and the method returns to operation 904 to consider a next Op.

If the Op is not present or absent in both of the Op log files, then the primary storage cluster determines if the Op is present (e.g., executed) on the primary storage cluster but not present on the secondary storage cluster at operation 910. If so, then the primary storage cluster replicates the Op to the secondary storage cluster at operation 912 and the method returns to operation 904.

If not, then the Op is undone by reading from the primary storage cluster and updating the Op log file of the secondary storage cluster and the method returns to operation 904 to obtain a next Op.

Operations 916, 918, and 920 may optionally occur in connection with operations 912 or 914. At operation 916, a dependent graph module will queue or dispatch an Op. At operation 918, the method includes performing a check (e.g., allocation check for allocation or allocation failure) on the Op with an overlapping write manager. At operation 920, the Op is sent to the secondary storage cluster.

Maintain the Benefit of Parallel Split of Ops to Primary and Secondary Storage Clusters in Synchronous Replication While Adding Support for Op Logging

When operations are sequentially split such that operations are first completed on a primary storage cluster and then completed on a secondary storage cluster, the active Op Log entries at a destination will be either a subset of Op log entries at a primary storage cluster or at best be the same as the Op log entries on the primary storage cluster. In one example, establishing a range of operations to be considered for persistent inflight tracking (IFT-P) replay can be a range of Ops on the Op log of the primary storage cluster.

In case of a parallel split of operations, an incoming Op (e.g., incoming WRITE operation) is sent to a synchronization replication circuitry (SR circuitry) in a primary storage cluster and a secondary storage cluster in parallel. Parallel splitting of Ops (e.g., Write Ops) improves performance of the Synchronous replication circuitry. By dispatching writes to local storage and its copies in parallel, the synchronous replication allows for fan-out factor of more than 1. Fan-out can be described as a topology where a single primary is associated with multiple secondary copies where Primary copy is Read-write and Secondary copies are Read-only. In this way, synchronous replication, using a parallel splitting based architecture, allows for more than 1 synchronous copy to be maintained without a linear increase in client latency or other overhead. However, this poses challenges from an Op logging perspective:

-   -   a. A range of operations for IFT-P replay needs to be         established. It is possible that a primary storage cluster is         ahead of the secondary storage cluster for the processing of the         operations, secondary storage cluster is ahead, or one of the         primary or second storage clusters has a subset of a range of         operations of the other storage cluster, etc.     -   b. Quick Reconcile (QR) procedure is invoked when a parallel         split WRITE operation fails on a primary storage cluster, but         succeeds on a secondary storage cluster. A quick reconcile         procedure involves undoing the WRITE operation on the secondary         storage cluster by bringing back old data from the primary         storage cluster. This can be a compound operation which may         include one or more WRITES, PUNCH HOLES or both. The quick         reconcile procedure needs to be Op logged to indicate its         completion so that during IFT-P replay, this QR procedure is         skipped if the QR procedure is already complete.

Various embodiments of the present technology provide for a wide range of technical effects, advantages, and/or improvements to stretched storage systems and participating distributed storage systems. For example, various embodiments may include one or more of the following technical effects, advantages, and/or improvements in case of a parallel split of operations:

-   -   a. The Op logs of the primary and secondary storage clusters         each maintain the active Op range of sequential numbers. Each Op         has a unique sequence number. During IFT-P replay, the ranges         from both the primary and secondary storage clusters will be         used to compare and establish the range for reconciliation.     -   b. A QR procedure will basically modify an already logged WRITE         which is being undone. A QR record will maintain subfields for         WRITE and Punch Hole and number of Ops needed to complete QR.         This record will be updated only by the last WRITE, and/or last         Punch Hole.

FIG. 10 is a flow diagram illustrating a set of operations for comparing ranges of Op logs for primary and secondary storage clusters while performing a persistent inflight tracking replay in accordance with an embodiment of the present disclosure. In the context of the present example, it is assumed a peered pair of CGs, including a local CG (e.g., CG 515 a) of a distributed storage system (e.g., cluster 510 a) and a peered remote CG (e.g., CG 515 b). One of the volumes of the local CG may have a failure and have an OOS state (e.g., OOS 620) for a period of time. Depending upon the nature of the data replication being performed (e.g., asynchronous or synchronous) and the particular implementation, a resynchronization may be performed responsive to any volume having OOS state.

State information regarding a data replication status of a mirror copy of a dataset associated with a local CG may be maintained, for example, to facilitate automatic triggering of resynchronization. For example, the state information may include information relating to the current availability or unavailability of a peer volume of a local CG.

At operation 1002, a computer implemented method 1000 establishes Op ranges for Op logs of the primary and second storage clusters.

Every Op or message that is split, will carry <HeadOpSeqNum, OpSeqNum> which basically is the SR circuitry's view of what is inflight. Both the Active Op Log of the primary storage cluster and Active Op Log of the secondary storage cluster know the <HeadOpSeqNum, TailOpSeqNum> which indicates the range of Ops that are potentially in the Log file. The Head Op Sequence Numbers indicates a first sequence number for a first Op and Tail Op Sequence Number indicates a last sequence number for a last Op within the range of Ops. Using the primary and secondary Op logs for the parallel split, it is possible to determine which Ops have to be replayed.

At operation 1004, the computer-implemented method compares the Op logs of the primary and secondary storage clusters. The Op-ranges from both of the primary and secondary Op logs can have the following relation between them:

-   -   A. If the ranges are the same for the Op logs, then at operation         1006 compare entries from a first entry to a last entry (e.g.,         Head to Tail) for primary and secondary Op logs and replay the         missing entries.     -   B. If one range is a subset of the other range for an Op log,         then at operation 1008 use the larger of the ranges to Compare         entries. Entries present on primary, but not on secondary will         need replication to the other side. Entries present on         secondary, but not on primary, will have to be undone on the         secondary storage cluster.     -   C. If the ranges intersect/overlap, then in this case at         operation 1010, the entries in a lower range up to the         intersection point have to be discarded by IFTP-replay because         these entries have been executed on both endpoints of primary         and secondary clusters, acknowledged back to SR circuitry, and         freed up by SR circuitry. The entries in a higher range will be         used for IFT-P replay.     -   D. If the ranges in the Op logs are disjoint, then in this case,         at operation 1012, the entries in a lower range have to be         discarded because they have been executed on both the endpoints,         acknowledged back and freed up by SR circuitry (e.g., splitter         component of SR circuitry). The entries in a higher range having         higher sequence numbers will be used for IFT-P resync. Depending         on which endpoint has the range with higher seq numbers, those         may have to be replicated to secondary storage cluster or undone         on the primary storage cluster.

The following are salient features of logging:

-   -   1. Every Op that is split will carry the following payload with         it: Op Sequence (Seq) Number, Head Seq Number(Num) as specified         by SR circuitry.     -   2. Since independent Ops are dispatched in parallel and executed         in parallel, an Op is allowed to update the <Head Op Seq Num,         Tail Op Seq Num> only if the Op Seq Num is >Tail Op Seq Num.     -   3. When an Op updates <Head Op Seq Num, Tail Op Seq Num>, only         the Op with Op Seq Num==Tail Op Seq Num is guaranteed to be         present in the Log file. It is possible that all Ops from (Tail         Op Seq Num-1) till (Head Op Seq Num) haven't been received and         hence not logged. IFTP-replay will have to determine valid         values from Active Op Log for replay.

In one example, an Op log file is a circular log file where an Op is inserted into Active Op Log at the index=(Op Seq Num % Max Entries). The following examples T0 to T12 in the below table 1 will clarify how range information in the Active Op Log file header will be put to use during replay: Consider Active Op Log of max size of 5 elements. In one example, parallel split Ops don't have a suffix, sequentially split Ops will have a ‘S’ suffix, Lun metadata Ops have a ‘L’ suffix, while successful QR will have a ‘Q’ suffix. Each snapshot shows the contents of the Op Log files and how the entries can be used for IFTP-replay.

TABLE 1 Primary Secondary, Src Op Head, Exp Op Dst Op Head, Exp Op Index Seq # Tail Range Seq# Tail Range Inference T0: Splitter has dispatches Ops 1-5, destination (dst) receives 2-3, source (src) receives 4 I1 # H <1-4> # H <1-3> One Op range is a subset of the other Op range. The IFT-P can run on the super set range of Ops <1-4> and carry out replay: <2-3> will be undone on the secondary. <4> will be applied to secondary. I1 # H <1-4> # H <1-3> I2 # 2 I3 # 3 T I4 4 T # I5 # # T1: Splitter dispatches no more Ops, src receives all 5, dst receives all but Op 1 I1 1 H 1-5 # H <1-5> The ranges are the I2 2 2 same. I3 3 3 The IFT-P resync will I4 4 4 run on the range of 1-5. I5 5 T 5 T 1 is applied to secondary. T2: The dst received Op1 and executed it. I1 1 H 1-5 1 H 1-5 The ranges are the I2 2 2 same. I3 3 3 The IFT-P resync will I4 4 4 run on the range of 1-5. I5 5 T 5 T T3: Splitter has freed up 1-4, has dispatched 6, 7, 8, 9. Dst receives only 9. Src hasn't received any I1 1 H <1-5> stale-1 <5-9> The dst range has I2 2 stale-2 moved forward. Skip I3 3 stale-3 1-4. IFT-P replay will I4 4 9 T run on range of 5-9. I5 5 T 5 H I1-I3 are expected to have Ops 6, 7 and 8, but I1-I3 don't. Therefore, the 3 entries will be skipped during IFT-P replay. 9 will be undone on secondary. T4: Src receives 6, 7, 8, 9. Dst receives 6, 7, 8 I1 6 <5-9> 6 I5-I4 The dst range and the I2 7 7 Ops<5-9> src range are the same. I3 8 8 IFT-P resync will be I4 9 T 9 T carried out on range I5 5 H 5 H Op5-Op9. T5: splitter receives acks for 5, 6, 7, 8, 9. dispatches only 10. Dst receives it, src doesn't yet I1 6 I5-I4 stale-6 I5-I5 The dst range has I2 7 Ops<5-9> stale-7 Ops<10-10> moved ahead. Op5- I3 8 stale-8 Op9 will be discarded I4 9 T stale-9 by IFTP-resync, but I5 5 H 10  H, T only Op10 will be considered and it will be undone at the secondary. T6- src receives 10 I1 stale-6 I5-I5 stale-6 I5-I5 There is only one I2 stale-7 Ops<10-10> stale-7 Ops<10-10> Op10 to be reconciled. I3 stale-8 stale-8 IFT-P resync uses that I4 stale-9 stale-9 range. I5 10  H, T 10  H, T T7: Seq split independent O MD Ops 11, 12 are executed on primary, A parallel-split Op 13 is dispatched. 13 is received on dst, but not on src I1 11S H <I1-I2> stale-6 H <I1-I3> The super-set range I2 12S-Dep-11 T Ops<11-12> stale-7 Ops<11-13> Op11-Op13 will be I3 stale-8 13  T used by IFT-P resync. I4 stale-9 stale-9 Op11, Op12 will be I5 stale-10 stale-10 replicated to secondary. Op13 will be undone on the secondary. T8: The MD Ops of 11, 12 are executed on both the endpoints. 13 is executed on src. I1 11S H <I1-I3> 11S H <I1-I3> The ranges in the I2 12S-Dep-11 Ops<11-13> 12S Ops<11-13> common set (11-13) I3 13  T 13  T will be used by IFT-P I4 stale-9 stale-9 resync. I5 stale-10 stale-10 T9 Splitter receives ack for 11, 12, 13 and dispatches an independent Op of 14, Lun Metadata Op 15 which only the src has executed, but not dst yet I1 stale-11S I4-I5 stale-11S H <I1-I3> The ranges Op14-15 I2 stale-12S-Dep-11 Ops<14-15> stale-12S-Dep-11 Ops<11-13> will be used for IFTP- I3 stale-13 stale-13 T replay while I4 14  H stale-9 discarding the older I5 15-LS T stale-10 range of Op11-13. T10: 14, 15-LS are received at dst. I1 stale-11S Ops<14-15> stale-11S Ops<14-15> The ranges 14-15 will I2 Stale-12S-Dep-11 Stale-12S-Dep-11 be used for IFT-P I3 stale-13 Stale-13 replay. I4 14  H 14  H I5 15-LS T 15-LS T T11: Splitter receives ack for 14, 15-LS; sends WRITEs 16 which goes through a QR, 17 which fails at both the end-points I1 stale-11S Ops<14-15> 16Q H, T Op<16-16> The range 16-16 will I2 Stale-12S-Dep-11 stale-12S-Dep-11 be used for IFT-P I3 stale-13 Stale-13 replay. Ops <14-15> I4 14  H Stale-14 will be skipped. Since I5 15-LS T Stale-15-LS 16 is already done with QR, it will be skipped too. T12: Splitter has received acks for both 16, 17. Now it issues 18S which has executed on primary, but not on secondary I1 Stale-11S Ops<18-18> 16Q H, T Op<16-16> The range 18-18 will I2 Stale-12S-Dep-11 Stale-12S-Dep-11 be used for IFT-P I3 18S H, T Stale-13 replay. Op 18 will be I4 Stale-14 Stale-14 replicated to dst. I5 Stale-15-LS Stale-15-LS

QR Procedure

Various features may be implemented for a synchronous replication relationship. In an example, a reconciliation (e.g., a quick reconciliation is performed within a data path and is done at a granularity of affected operations, while maintaining synchronous copies, such as replication storage objects, in a synchronous state) may be performed in response to an error (e.g., an I/O operation may fail to be implemented by the first storage cluster or controller upon a first storage object, thus causing a data divergence), and thus the reconciliation may be performed to make the first storage object and the replication storage objects data consistent. The reconciliation may be a quick reconciliation that is relatively faster than alternate approaches that involve taking the synchronous replication relationship out-of-sync and working through a costly resync operation to achieve reconciliation.

FIG. 11 is a flow diagram illustrating a set of operations for performing a quick reconciliation in accordance with an embodiment of the present disclosure. In the context of the present example, it is assumed a peered pair of CGs, including a local CG (e.g., CG 515 a) of a distributed storage system (e.g., cluster 510 a) and a peered remote CG (e.g., CG 515 b). One of the storage objects (e.g., data containers, volumes) of the local CG may have a failure and have an OOS state (e.g., OOS 620) for a period of time. Depending upon the nature of the data replication being performed (e.g., asynchronous or synchronous) and the particular implementation, a resynchronization may be performed responsive to any storage object having OOS state.

At operation 1102, a computer implemented method 1100 includes initiating a quick reconcile having compound operations (e.g., read, write, deallocate space Op (e.g., punch hole Op)) when a parallel split operation fails on a first storage object of a primary storage cluster and succeeds on a second storage object (e.g., a replicated storage object) of a secondary storage cluster. A quick reconciliation is performed within a data path and is done at a granularity of affected operations, while maintaining synchronous copies, such as replication storage objects, in a synchronous state to avoid a resynchronization process.

The deallocate space Op deallocates space in a byte range starting at offset and continuing for length bytes. Within the specified range, partial filesystem blocks are zeroed, and whole filesystem blocks are removed from the file. After a successful call, subsequent reads from this range will return zeroes. At operation 1104, the method includes initiating an undo of a write Op (e.g., undo a number of blocks of the write Op with the blocks including version 2 of data) at the secondary storage cluster because the write Op failed on the primary storage cluster. The undo occurs via a read Op of the data (e.g., version 1 of data, old content) at the primary storage cluster and applies this data to the secondary storage cluster with a write Op. Since the undo is completed at end of QR, its completion needs to be recorded with logging at operation 1106. The record at the secondary storage cluster can be deleted or mark the earlier recorded Op as QR. The QR record will maintain subfields for write Op and deallocate space Op and number of Ops needed to complete QR. The QR record will be updated only by the last WRITE Op and/or last deallocate space Op (e.g., PUNCH-Hole Op).

Consider a case where a successful QR is followed by a delete of the LUN or truncate of the LUN. In such a case, QR induced read Op is taught to deal with this failure. Instead, logging QR avoids IFT-P replay from repeating a QR, which is intuitive and maintenance friendly.

A QR can result in either one or both of replicating Ops (e.g., Push data, Push hole) being performed depending on whether data or holes (absence of data) exist in a primary file range. A replicating Op has a special flag citing that the Op is a QR. A replicate Op handler passes a new file system flag with a last file system message so that the Op log registers a successful QR. The replicating Ops include a QR message count to specify a number of replicating Ops that are needed to complete the QR. One or more file system messages may be needed depending on a number of ranges represented by the replicating op. A last or most recent message from Push data or Push hole updates previous write Op records with QR information. If the sum of data count and hole count value is equal to incoming QR message count, then the log record is marked as QR done. During a Log replay, the messages from the two replicating Ops may be replayed out of order since the ranges are disjoint. However, pending messages in the Log are replayed.

During an IFT-P replay, write Ops which are committed only onto the secondary storage cluster will have to be undone by reading from the primary storage cluster and applying the same data from the primary storage cluster to the secondary storage cluster. This is very much like a QR which is encountered during the steady state. Similar to the steady state, the undo will read the old data from primary storage cluster and apply the same data at the secondary storage cluster. The Op will be logged in Op log of the secondary storage cluster as a successful completed QR like in the steady state.

Early Engagement of Op Logging While Marking the Op Log File Usable Later to Avoid User IO Latency

Op logging is relevant when the primary and secondary storage clusters are in sync state of replication. If Op logging is implemented by pausing all I/O just before entering in sync state, start Op logging and resume user I/O, then this will cause a spike in user I/O latency. The present design maintains the benefit of a transition state while adding Op logging support. The transition state or phase transitions from asynchronous to synchronous (or instantiates synchronous session) for the relation between primary and secondary storage clusters. This present design starts Op logging early and marks the Op Log as valid once a relation between a first storage object of the primary storage cluster and a replicated second storage object of the secondary storage cluster transitions from OOS state to in sync state. A metadata log replay can play Ops out of order at destination from an Op Log sequence number point of view. The persistent inflight tracker logic will skip Op logging of Ops that are below an Active Op Range.

FIG. 12 is a flow diagram illustrating a set of operations for early engagement of Op logs for primary and secondary storage clusters while marking the Op log file usable later to reduce user I/O latency in accordance with an embodiment of the present disclosure. In the context of the present example, it is assumed a peered pair of CGs, including a local CG (e.g., CG 515 a) of a distributed storage system (e.g., cluster 510 a) and a peered remote CG (e.g., CG 515 b). One of the storage objects of the local CG may have a failure and have an OOS state (e.g., OOS 620) for a period of time. Depending upon the nature of the data replication being performed (e.g., asynchronous or synchronous) and the particular implementation, a resynchronization may be performed responsive to any storage object having OOS state at least as an internal event.

State information regarding a data replication status of a mirror copy of a dataset associated with a local CG may be maintained, for example, to facilitate automatic triggering of resynchronization. For example, the state information may include information relating to the current availability or unavailability of a peer storage object of a local CG.

A splitter component of SR circuitry goes through various sub-phases to allow a smooth transition from async replication to sync replication. Depending on the sub-phase, a decision for Op logging or no Op logging will occur. The following table 2 explains the various sub-phases of the splitter component for transitioning from async replication to sync replication.

Sub-phase Op logging for Metadata Ops Op logging for data Ops DRL sub-phase Metadata Ops are Op logged on a Data Ops are not Op logged primary storage cluster and also enter on the primary storage cluster. a Metadata log (MDL) journal. The data Ops execute on the primary storage cluster with changes being tracked in a DRL bitmap. MDL Drain sub- Ops from the MDL journal are Data Ops are not Op logged phase replicated to the secondary storage on primary storage cluster. cluster. These Ops now Op log at The data Ops execute on secondary storage cluster. primary storage cluster with changes tracked in DRL bitmap. DRL drain sub- Metadata Ops are executed on Data Ops which are phase primary storage cluster and Op logged independent of objects at primary storage cluster. tracked in DRL are executed Metadata Ops are replicated to on primary storage cluster and secondary storage cluster and Op Op logged at the primary logged at the secondary storage storage cluster. The data Ops cluster. are replicated to secondary storage cluster and Op logged at the secondary storage cluster. Splitting sub-phase Metadata Ops are executed at primary Data ops are executed at and secondary storage clusters and Op primary and secondary logged at primary and secondary storage clusters and Op storage clusters. logged at primary and secondary storage clusters.

FIG. 12 illustrates the various sub-phases of the splitter component.

At operation 1202, a computer implemented method 1200 determines whether a sub-phase (e.g., dirty region logging (DRL) sub-phase, MDL drain sub-phase to drain metadata log, DRL drain phase sub-phase (also referred to as cut over split phase), splitting sub-phase) of a splitter component causes Op logging. If so, then at operation 1204 the method starts Op logging early and engages the splitter component of SR circuitry before a synchronous replication relationship between a storage object of the primary storage cluster and a replicated storage object of the secondary storage cluster transitions from OOS state to in sync state. After the splitter component is engaged and before the synchronous replication relationship enters in sync state, the following can happen to incoming Ops depending on the sub-phase of the splitter component.

At operation 1206, for DRL sub-phase, metadata Ops (e.g., Create, Link, Unlink, Rename, Set Attribute, Open, Close) are executed and op logged on the primary storage cluster and entered into a metadata log (MDL) journal of the primary storage cluster. At operation 1207, for a MDL drain sub-phase to drain metadata log, these metadata Ops from the MDL journal will be replicated later to secondary storage cluster by metadata scan process and enter Op log at the secondary storage cluster.

At operation 1208, for a DRL drain sub-phase, metadata Ops are executed on primary storage cluster and replicated to secondary storage cluster as well. The metadata Ops are logged into Op log of primary storage cluster and replicated Ops enter Op log of secondary storage cluster as well. For DRL drain, the data ops that are independent of the regions marked in DRL and independent of DRL scan will be Op logged on the primary storage cluster. The data ops are replicated to secondary storage cluster when replication occurs and then Op log these data Ops on secondary storage cluster.

At operation 1210, for a splitting sub-phase, metadata Ops are executed at primary and secondary storage clusters and Op logged at primary and secondary storage clusters. Data ops are executed at primary and secondary storage clusters and Op logged at primary and secondary storage clusters.

At operation 1212, given a splitter sub-phase that does not cause Op logging of data Ops, for DRL or MDL drain sub-phases, data Ops (e.g., write, punch holes) are executed on the primary storage cluster with changes being tracked in a DRL bitmap. The data Ops are not logged into the Op log of the primary storage cluster.

A dirty region log is used to track regions within the storage object that are modified by data operations, such as write operations executed during a last incremental transfer. The dirty region log may comprise bits that can be set to either a dirty indicator or a clean indicator. A bit may be mapped to a region within the storage object. Thus, the bit can be set to the dirty indicator to indicate that a data operation has modified the region (e.g., the region now comprises data not yet replicated to the replicated storage object). The bit can be set to the clean indicator to indicate that the region is now clean (e.g., the region has not been modified with data not yet replicated to the replicated storage object, and thus the region comprises the same data as a corresponding region within the replicated storage object).

A metadata log is used to track metadata operations that modify the storage object, such as a create operation (e.g., a create file operation, a create LUN operation, etc.), a link operation, an unlink operation, a rename operation (e.g., a file rename operation, etc.), a set attribute operation (e.g., a set volume size operation, an assign permissions operation, etc.), etc. In one example, copies of metadata operations executed upon the storage object during the last asynchronous transfer are inserted into the metadata log.

The computer implemented method marks the Op log usable once the synchronous replication relationship between a storage object of a primary storage cluster and a replicated storage object of a secondary storage cluster enters in sync state.

Although the operations in the computer-implemented methods disclosed herein are shown in a particular order, the order of the actions can be modified. Thus, the illustrated embodiments can be performed in a different order, and some operations may be performed in parallel. Some of the operations listed in the methods disclosed herein are optional in accordance with certain embodiments. The numbering of the operations presented is for the sake of clarity and is not intended to prescribe an order of operations in which the various operations must occur. Additionally, operations from the various flows may be utilized in a variety of combinations.

In one example, the operations 1206, 1207, 1208, and 1208 are illustrated as separate parallel operations. In another example, these operations can occur sequentially such as operation 1207 being performed after operation 1206.

Maintain the Benefits of Transition State While Coming to In Sync State

It would be trivial to start Op Logging while only in the In Sync state for a synchronous replication relationship between a first storage object of the primary storage cluster and a replicated second storage object of the secondary storage cluster. Conventional techniques for transitioning from asynchronous replication to synchronous replication must pause client I/O (e.g., stop, block, fail, or queue the client I/O for later execution), which increases latency (e.g., increased latency while the client I/O is queued). This also affects the operation of client devices accessing data within the existing volume (e.g., an application may timeout or experience errors when data access operations, attempting to access data, are blocked or failed).

Accordingly, methods and/or systems are provided herein that can transition a storage object from an asynchronous replication state or other non-synchronous state (e.g., an out of sync state) to a synchronous replication state in a manner that mitigates client disruption and latency. However, this will need an explicit differentiation between transition state and the In Sync state and this can cause a latency spike in the user I/O into a transition state. Thus, the method 1100 starts Op logging early and marks the Op Log as usable once the synchronous replication relationship enters into the In Sync state.

A metadata log replay can play Ops out of order at destination from an Op Log seq number point of view.

FIG. 13 illustrates a synchronous replication (SR) system with tracker updates for a transition or resynchronization process in accordance with one embodiment. The SR system 1300 includes a splitter component 1320 (or replicating circuitry) and a primary file system 1330 that are located in a primary storage cluster while a writer 1340 and a secondary file system 1350 are located in a secondary storage cluster. At operation 2, an Op 1310 is queued in a queue 1316 of splitter component 1320 after being sent to dependent graph at operation 1 and then passing (or failing) dependent graph 1312 and overlapping write manager 1314 checks. An entry for the Op is added to the queue 1316. The splitter component 1320 will determine a number of consecutive entries with state as responded, shrink a size of splitter active Ops by incrementing a head, fill a local file system message with additional details of <Head Op Seq#, Op Seq#>, and fill a replicated Op with the above details. Next, the Op is sent to the file system 1330 at operation 3, and the file system 1330 computes a location for a record of the Op, inserts the Op, updates a log marker if permitted, and deletes any older entries in the Op log of the primary storage cluster if these entries are not needed.

Subsequently, at operation 4, the splitter component 1320 will update the record state in splitter active Ops as responded for the Op.

At operation 3′ the replicated Op is sent to writer 1340 to be executed on a replicated storage object. The replicated Op is provided into a record of file system 1350, which also computes a location for a record of the replicated Op, inserts the replicated Op, updates a log marker if permitted, and deletes any older entries in the Op log if these entries are not needed. At operation 4′, the splitter component 1320 will update the record state in splitter active Ops as responded for the replicated Op. Responses for operations 5 and 6 complete the transition or resynchronization process.

It is important to assign sequence numbers after Dependent Graph and Overlapping Write Manager checks have been cleared. The following example highlights the reason by showing what happens if the above rule wasn't followed:

-   -   1. Consider two operations truncate of F1, expand of F1 coming         in from two different threads.     -   2. If the above rule was not followed and seq numbers 1 was         assigned to Truncate and 2 for expand before even they went         through Dependent Graph checks.     -   3. If the actual order of execution on a filesystem (e.g., write         anywhere file system (WAFL)) was Seq 2 and Seq1, then the Ops         are logged in Op Log of the primary storage cluster. The result         of the above Ops at the primary storage cluster is that the file         is truncated.     -   4. Consider if the secondary storage cluster crashes before         either of the Ops could be executed on it.     -   5. IFT-P replay will try to replicate Ops in the order Seq1 and         then Seq 2 (different order than actual order of execution on a         filesystem). The result at secondary storage cluster is an         expanded file. Thus, the primary and secondary storage clusters         have different resulting files if sequence numbers are not         assigned after Dependent Graph and Overlapping Write Manager         checks have been cleared.

FIG. 14 illustrates parallel splitting of data Ops and sequentially splitting of modifying metadata Ops for a multi-site distributed storage system having synchronous replication (SR) between primary and secondary sites in accordance with one embodiment. The multi-site distributed storage system 1400 includes a splitter component 1420 (or replicating circuitry) and a primary file system 1430 that are located in a primary storage cluster 1410 while a writer 1460 and a secondary file system 1470 are located in a secondary storage cluster 1450. The granularity of protection and failover is a consistency group, which is a plurality of volumes that together host application data.

Initially, operations are received and handled by a protocol service 1412 (e.g., NAS service, SAN service) of the primary cluster 1410 and then the operations can be queued in a queue 1422 of splitter component 1420. The operations may represent a data operation (e.g., WRITE or PUNCH HOLE) or a metadata operation (e.g., CREATE, OPEN, RENAME, LINK, UNLINK, SETATTR, etc.) to be executed on a file associated with a volume hosted by a node of a storage cluster (e.g., primary storage cluster 1410, secondary storage cluster 1450). In one embodiment, by convention, operations to be executed in relation to a particular member volume of a CG may be directed to the monarch node and performed by an SVM (e.g., SVM 511 a or 511 b).

An entry for the Op is added to the queue. The protocol service 1412 or splitter component 1420 will determine whether the Op is a data operation or a metadata operation to be executed on a file. If a data operation is being processed, then a parallel split process 1421 causes the data operation to be sent along paths 1425 and 1426 to the file systems 1430 and 1470, respectively, in parallel simultaneously. Next, the file system 1430 computes a location for a record of the data operation, inserts the data operation, updates a log marker if permitted, and deletes any older entries in the Op log of the primary storage cluster if these entries are not needed.

The splitter component 1420 will update the record state in splitter active Ops as responded for the data operation. The replicated data operation is sent to writer 1460 to be executed on a replicated storage object. The replicated data Op is provided into a record of file system 1470, which also computes a location for a record of the replicated Op, inserts the replicated Op, updates a log marker if permitted, and deletes any older entries in the Op log if these entries are not needed. Subsequently, the result processor 1424 will receive updates for the data operation along paths 1431 and 1432. The record state in splitter active Ops are updated as responded for the data operation. The result processor 1424 sends updates to the protocol service 1412.

If a metadata operation is being processed, then a sequential split process 1445 causes the data operation to be sent initially to the file system 1430 and then sequentially to the file system 1470. The splitter component 1420 will update the record state in splitter active Ops as responded for the data operation. The replicated data operation is sent to writer 1460 to be executed on a replicated storage object. Subsequently, the result processor 1424 will receive an update for the data operation along sequential path 1435. The record state in splitter active Ops are updated as responded for the data operation. The result processor 1424 sends updates to the protocol service 1412.

Example Computer System

Embodiments of the present disclosure include various steps, which have been described above. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a processing resource (e.g., a general-purpose or special-purpose processor) programmed with the instructions to perform the steps. Alternatively, depending upon the particular implementation, various steps may be performed by a combination of hardware, software, firmware and/or by human operators.

Embodiments of the present disclosure may be provided as a computer program product, which may include a non-transitory machine-readable storage medium embodying thereon instructions, which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, fixed (hard) drives, magnetic tape, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), and magneto-optical disks, semiconductor memories, such as ROMs, PROMs, random access memories (RAMs), programmable read-only memories (PROMs), erasable PROMs (EPROMs), electrically erasable PROMs (EEPROMs), flash memory, magnetic or optical cards, or other type of media/machine-readable medium suitable for storing electronic instructions (e.g., computer programming code, such as software or firmware).

Various methods described herein may be practiced by combining one or more non-transitory machine-readable storage media containing the code according to embodiments of the present disclosure with appropriate special purpose or standard computer hardware to execute the code contained therein. An apparatus for practicing various embodiments of the present disclosure may involve one or more computers (e.g., physical and/or virtual servers) (or one or more processors within a single computer) and storage systems containing or having network access to computer program(s) coded in accordance with various methods described herein, and the method steps associated with embodiments of the present disclosure may be accomplished by modules, routines, subroutines, or subparts of a computer program product.

FIG. 15 is a block diagram that illustrates a computer system 1500 in which or with which an embodiment of the present disclosure may be implemented. Computer system 1500 may be representative of all or a portion of the computing resources associated with a node (e.g., storage nodes 136 a-n or storage nodes 146 a-n) of a distributed storage system (e.g., cluster 235 or 245) or an administrative workstation (e.g., computer system 110 or 210). Notably, components of computer system 1500 described herein are meant only to exemplify various possibilities. In no way should example computer system 1500 limit the scope of the present disclosure. In the context of the present example, computer system 1500 includes a bus 1502 or other communication mechanism for communicating information, and a processing resource (e.g., a hardware processor 1504) coupled with bus 1502 for processing information. Hardware processor 1504 may be, for example, a general purpose microprocessor.

Computer system 1500 also includes a main memory 1506, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1502 for storing information and instructions to be executed by processor 1504. Main memory 1506 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1504. Such instructions, when stored in non-transitory storage media accessible to processor 1504, render computer system 1500 into a special-purpose machine that is customized to perform the operations specified in the instructions.

Computer system 1500 further includes a read only memory (ROM) 1508 or other static storage device coupled to bus 1502 for storing static information and instructions for processor 1504. A storage device 1510, e.g., a magnetic disk, optical disk or flash disk (made of flash memory chips), is provided and coupled to bus 1502 for storing information and instructions.

Computer system 1500 may be coupled via bus 1502 to a display 1512, e.g., a cathode ray tube (CRT), Liquid Crystal Display (LCD), Organic Light-Emitting Diode Display (OLED), Digital Light Processing Display (DLP) or the like, for displaying information to a computer user. An input device 1514, including alphanumeric and other keys, is coupled to bus 1502 for communicating information and command selections to processor 1504. Another type of user input device is cursor control 1516, such as a mouse, a trackball, a trackpad, or cursor direction keys for communicating direction information and command selections to processor 1504 and for controlling cursor movement on display 1512. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.

Removable storage media 1540 can be any kind of external storage media, including, but not limited to, hard-drives, floppy drives, IOMEGA® Zip Drives, Compact Disc-Read Only Memory (CD-ROM), Compact Disc-Re-Writable (CD-RW), Digital Video Disk-Read Only Memory (DVD-ROM), USB flash drives and the like.

Computer system 1500 may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware or program logic which in combination with the computer system causes or programs computer system 1500 to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system 1500 in response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506. Such instructions may be read into main memory 1506 from another storage medium, such as storage device 1510. Execution of the sequences of instructions contained in main memory 1506 causes processor 1504 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions.

The term “storage media” as used herein refers to any non-transitory media that store data or instructions that cause a machine to operation in a specific fashion. Such storage media may comprise non-volatile media or volatile media. Non-volatile media includes, for example, optical, magnetic or flash disks, such as storage device 1510. Volatile media includes dynamic memory, such as main memory 1506. Common forms of storage media include, for example, a flexible disk, a hard disk, a solid state drive, a magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge.

Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1502. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications.

Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor 1504 for execution. For example, the instructions may initially be carried on a magnetic disk or solid state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1500 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 1502. Bus 1502 carries the data to main memory 1506, from which processor 1504 retrieves and executes the instructions. The instructions received by main memory 1506 may optionally be stored on storage device 1510 either before or after execution by processor 1504.

Computer system 1500 also includes a communication interface 1518 coupled to bus 1502. Communication interface 1518 provides a two-way data communication coupling to a network link 1520 that is connected to a local network 1522. For example, communication interface 1518 may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1518 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1518 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

Network link 1520 typically provides data communication through one or more networks to other data devices. For example, network link 1520 may provide a connection through local network 1522 to a host computer 1524 or to data equipment operated by an Internet Service Provider (ISP) 1526. ISP 1526 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet” 1528. Local network 1522 and Internet 1528 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1520 and through communication interface 1518, which carry the digital data to and from computer system 1500, are example forms of transmission media.

Computer system 1500 can send messages and receive data, including program code, through the network(s), network link 1520 and communication interface 1118. In the Internet example, a server 1530 might transmit a requested code for an application program through Internet 1528, ISP 1526, local network 1522 and communication interface 1518. The received code may be executed by processor 1104 as it is received, or stored in storage device 1510, or other non-volatile storage for later execution. 

What is claimed is:
 1. A computer implemented method performed by one or more processing resources of a distributed storage system, the method comprising: determining whether a sub-phase of a splitter component of a synchronous replication (SR) circuitry causes Op logging; starting Op logging in an Op log early during a transition state from asynchronous replication to synchronous replication to avoid input/output latency when the sub-phase of the splitter component causes Op logging; and engaging the splitter component of synchronous replication (SR) circuitry before a synchronous replication relationship between a storage object of the first cluster and a replicated storage object of the second cluster transitions from an out of sync (OOS) state to an in sync state.
 2. The computer implemented method of claim 1, wherein after the splitter component is engaged and before the synchronous replication relationship enters in sync state for a dirty region log (DRL) sub-phase, metadata Ops are executed and Op logged on the primary storage site and entered into a metadata log journal of the primary storage site.
 3. The computer implemented method of claim 1, wherein the metadata Ops are replicated to the secondary storage site by a metadata scan process and enter an Op log at the secondary storage site.
 4. The computer implemented method of claim 1, wherein for a dirty region log (DRL) drain or splitting sub-phase, metadata Ops are executed on the primary storage site and replicated to the secondary storage site, wherein the metadata Ops are logged into an Op log of the primary storage site and replicated Ops enter an Op log of secondary storage site.
 5. The computer implemented method of claim 1, wherein for a dirty region log (DRL) drain, data ops that are independent of the regions marked in a dirty region log will be Op logged on the primary storage site, wherein the data ops are replicated to the secondary storage site and then Op log these data Ops on the secondary storage site.
 6. The computer implemented method of claim 1, wherein for a splitting sub-phase, data ops are executed on the primary storage site and Op logged on the primary storage site as part of execution, wherein the data Ops are replicated to the secondary storage site and Op logged on secondary storage site.
 7. The computer implemented method of claim 1, wherein given a splitter sub-phase that does not cause Op logging of data Ops, data Ops are executed on the primary storage site and tracked with a dirty node region log (DRL), which tracks data changes, wherein the data Ops are not logged into the Op log of the primary storage site.
 8. The computer implemented method of claim 1, wherein the processing resource is configured to execute instructions to mark the Op log usable once the synchronous replication relationship between the storage object of the first cluster and the replicated storage object of the second cluster enters in sync state.
 9. A non-transitory computer-readable storage medium embodying a set of instructions, which when executed by a processing resource of a multi-site distributed storage system cause the processing resource to: determine whether a sub-phase of a splitter component of a synchronous replication (SR) circuitry causes Op logging; start Op logging in an Op log early during a transition state from asynchronous replication to synchronous replication to avoid input/output latency when the sub-phase of the splitter component causes Op logging; and engage the splitter component of synchronous replication (SR) circuitry before a synchronous replication relationship between a storage object of the first cluster and a replicated storage object of the second cluster transitions from an out of sync (OOS) state to an in sync state.
 10. The non-transitory computer-readable storage medium of claim 9, wherein after the splitter component is engaged and before the synchronous replication relationship enters in sync state for a dirty region log (DRL) sub-phase, metadata Ops are executed and Op logged on the primary storage site and entered into a metadata log journal of the primary storage site.
 11. The non-transitory computer-readable storage medium of claim 9, wherein the metadata Ops are replicated to the secondary storage site by a metadata scan process and enter an Op log at the secondary storage site.
 12. The computer implemented method of claim 9, wherein for a dirty region log (DRL) drain or splitting sub-phase, metadata Ops are executed on the primary storage site and replicated to the secondary storage site, wherein the metadata Ops are logged into an Op log of the primary storage site and replicated Ops enter an Op log of secondary storage site.
 13. The computer implemented method of claim 9, wherein for a dirty region log (DRL) drain, data ops that are independent of the regions marked in a dirty region log will be Op logged on the primary storage site, wherein the data ops are replicated to the secondary storage site and then Op log these data Ops on the secondary storage site.
 14. The computer implemented method of claim 9, wherein for a splitting sub-phase, data ops are executed on the primary storage site and Op logged on the primary storage site as part of execution, wherein the data Ops are replicated to the secondary storage site and Op logged on secondary storage site.
 15. A multi-site distributed storage system having a primary storage site with a first cluster and a secondary storage site with a second cluster comprising: a processing resource; and a non-transitory computer-readable medium coupled to the processing resource, having stored therein instructions, which when executed by the processing resource cause the processing resource to: determine whether a sub-phase of a splitter component of a synchronous replication (SR) circuitry causes Op logging; start Op logging in an Op log early during a transition state from asynchronous replication to synchronous replication to avoid input/output latency when the sub-phase of the splitter component causes Op logging; and engage the splitter component of synchronous replication (SR) circuitry before a synchronous replication relationship between a storage object of the first cluster and a replicated storage object of the second cluster transitions from an out of sync (OOS) state to an in sync state.
 16. The multi-site distributed storage system of claim 15, wherein after the splitter component is engaged and before the synchronous replication relationship enters in sync state for a dirty region log (DRL) sub-phase, metadata Ops are executed and Op logged on the primary storage site and entered into a metadata log journal of the primary storage site.
 17. The multi-site distributed storage system of claim 15, wherein the metadata Ops are replicated to the secondary storage site by a metadata scan process and enter an Op log at the secondary storage site.
 18. The multi-site distributed storage system of claim 15, wherein for a dirty region log (DRL) drain or splitting sub-phase, metadata Ops are executed on the primary storage site and replicated to the secondary storage site, wherein the metadata Ops are logged into an Op log of the primary storage site and replicated Ops enter an Op log of secondary storage site.
 19. The multi-site distributed storage system of claim 15, wherein for a dirty region log (DRL) drain, data ops that are independent of the regions marked in a dirty region log will be Op logged on the primary storage site, wherein the data ops are replicated to the secondary storage site and then Op log these data Ops on the secondary storage site.
 20. The multi-site distributed storage system of claim 15, wherein for a splitting sub-phase, data ops are executed on the primary storage site and Op logged on the primary storage site as part of execution, wherein the data Ops are replicated to the secondary storage site and Op logged on secondary storage site. 